<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>关灯</button>
</body>

</html>
<script>
    var btn = document.querySelector('button');
    var flag = 0
    btn.onclick = function () {
        if (flag == 0) {
            document.body.style.backgroundColor = 'black'
            this.innerHTML = '开灯'
            flag = 1
        } else {
            document.body.style.backgroundColor = 'white'
            this.innerHTML = '关灯'
            flag = 0
        }
    }
</script>